Someone I know wants to trade their's for one of my guns. I know they're not a robust go to war gun and also know about the multimag options. Thinking it might be a nice stow in the truck gun being able to pair with a handgun and reach out a bit more with 9mm. But how about your hands on experience- reliability, accuracy etc?

I've had one for several years and no issues with performance or reliability.  Shoots straight and eats whatever ammo i feed it (though i've never shot steel cased).  I've used it for Steel Challenge/Knowckdown shooting and at the range.

If you get one there are two things you'll change.  1 - It has more recoil than you'd think so you'll want to add some kind of pad to the buttstock.  2 - the stock trigger is a plastic piece of garbage.  It's sharp and it flexes, so replace it with an aluminum trigger - https://www.mcarbo.com/blem-sub-2000-curved-trigger-blk.aspx

From there it's good to go.

Lots of people add an optic, larger charging handle, etc, etc, but IMO that's a bit like adding lipstick on a pig...

I have a Kel Tec sub2000 Gen 2. It is the glock 10 mag model. I love it. The gun is great. Like 124gr9mm said the gun does have more recoil than you imagine. I added a buffer tube cover and a brake to mine. The gun is more pleasant to shoot now. 

I've had one for several years, and I love it.  It's accurate, reliable, and fun to shoot.  But as has been noted, you will probably need some mods. I found it extremely difficult to get my head low enough to use the iron sights, so I had to add an RDS, which helps a lot.  But then you need a mount that will still allow you to fold the gun.  I would be happy to provide more specifics, if you like.